The present application relates generally to lighting devices. While it finds particular application to lighting devices employing one or more light-emitting diodes (LED).
Light-emitting diodes (LEDs) have been used in various light devices. In one such application, a flashlight has included a plurality of batteries connected electrically in series with a fixed, current-limiting resistor, an LED, and a switch that opens and closes the circuit. With the circuit so configured, the diode forward current varies as a function of both the battery voltage and the diode forward voltage.
However, batteries are generally characterized by a sloping discharge curve, with their output voltage decreasing as the batteries discharge. While the value of the resistor can be selected to provide a desired diode forward current when the batteries are fully charged, the current will decrease as the batteries discharge, and energy that could otherwise be used to produce useful illumination is dissipated in the resistor. The value of the resistor can also be selected to provide the desired forward current at a point relatively lower on the discharge curve. While doing so tends to reduce the power dissipated in the resistor, the diode forward current will be greater than desired when the batteries are more fully charged. Such an approach is likewise relatively inefficient, and can result in greater than desired diode power dissipation.
According to another approach, a switching regulator circuit configured as a current regulator has been used to drive one or more LEDs at a substantially constant forward current. While such an approach can provide improved current regulation compared to the use of a fixed current-limiting resistor, it also tends to be relatively expensive, and the switching regulator circuit and its associated circuitry can be bulky. Moreover, losses in the switching regulator circuit can have a deleterious effect on the overall efficiency.